Learn About Cell Division
2.5 Cell Division
2.5.4 Describe the events that occur in the four phases of mitosis (prophase, metaphase, anaphase and telophase.
2.5.4 Describe the events that occur in the four phases of mitosis (prophase, metaphase, anaphase and telophase (2).
Describe means to give a detailed account.
Super coiling: Eukaryotic DNA is combined with histone proteins and non-histone proteins to form chromatin. The method of folding of chromatin is specific to each chromosome leaving genes in predictable positions and a distinctive overall chromosome shape. The human cell has a DNA length of about 1.8 m this has to be packed into a nucleus which has only a 5 um diameter. This packaging process requires up to a X 15,000 reduction. This super coiling makes the structure so dense that it can be see with a light microscope during the phases of mitosis.
In this sequence only one chromosome is illustrated so that we can more clearly follow the process. In a human a complete diagram would have 46 chromosomes each replicating and condensing and separating.
a)The cell membrane is intact during this the interphase. The chromosomes cannot be seen during G1,S and G2.
b) G1,Within the nucleus, genes on the chromosome are being expressed to carry out normal cell function (interphase). Remember you cannot see chromosomes at this stage. The diagram has a 'see's through' the nuclear membrane so you can see inside. In reality it would look just like cell a).
c) S-phase in which DNA replication occurs and the chromosomes are copied. The copies called sister chromatids are held together by a protein to form the centromere. It is still not possible to see this happen with an intact cell.
d) Early Prophase in which the sister chromatids have condensed by super coiling. Note the formation of the spindle microtubules and their attachment to centrioles. The nuclear membrane will now break down to reveal sister chromatids. The internal arrangements of chromosomes can now be seen with a light microscope.
e) Metaphase the chromosomes arranged on the equator of the cell each attached to a spindle microtubule at the centromere
f) Anaphase: The spindle microtubules contract and pull apart the sister chromatids one to each pole of the cell. The centromere splits allowing the sister chromatids to be separate.
g) Telophase: at each pole there are separate groups of the replicated chromosomes the spindles is degenerating
h) Cytokinesis: the cell membrane begins to separate, dividing the cell into two new cells. The nuclear membrane is reforming around each cell.
i) Two daughter cells are formed. They are genetically identical to each other and in effect the basis of a clone. (see 2.5.6)
Notice that cell a) begins with one chromosome and that by step h) there are two cells each with a copy of that chromosome.
As suggested by cell theory, all cells have come from other cells.
Much of the information and images adapted from Click4Biology.